The present invention relates to a method of efficiently separating and recovering volatile ruthenium contained in high-level radioactive liquid waste discharged from the reprocessing step of spent nuclear fuel by purex process.
In the reprocessing step of spent nuclear fuel by purex process, there is utilized a method wherein uranium and plutonium are extracted with a solvent from a solution of spent fuel in nitric acid and leaving radioactive fission products in the solution of nitric acid. The solution of fission products in nitric acid is referred to as high-level radioactive liquid waste (hereinafter abbreviated to "high-level liquid waste"), which is eventually solidified, stored and disposed of as a vitrified body. When the high-level liquid waste is melt mixed with a vitrifying agent in a melting furnace in the vitrifying treatment, ruthenium as one of the fission products interferes with the stable operation of the glass melting furnace.
Under such circumstances investigations have heretofore been made on the separation and removal of ruthenium from the high-level liquid waste. As one of the measures, there is proposed a method wherein the high-level liquid waste is electrolyzed at a constant potential to electrolytically oxidize ruthenium.
However, the ruthenium which is formed by the above-mentioned electrolytic oxidation is volatile and is separated in gaseous form from the high-level liquid waste. The gaseous ruthenium is chemically unstable, is liable to adhere to the walls of surrounding equipments, and accelerates the corrosion of the process equipments by its oxidizing power. Since ruthenium as one of the fission products is a radioactive substance, it pollutes the equipments when adhered thereto. In addition, volatile ruthenium is unlikely to be absorbed in water and worsens the efficiency of its removal in a scrubber, thus increasing the amount of the radioactive ruthenium discharged to the environment. Even when the volatile ruthenium is absorbed in an absorbent, the amount of the ruthenium thus absorbed is limited because of the poor absorbing performance of the absorbent, thus increasing the amount of radioactive solid waste.